Hydrothermal Reactions under Supercritical Conditions. II. The Reaction between Calcium Hydroxide and Silica

1957 ◽  
Vol 61 (7) ◽  
pp. 941-943 ◽  
Author(s):  
J F. Corwin ◽  
R. G. Yalman ◽  
J. W. Edwards ◽  
E. R. Shaw
Keyword(s):  
Calcium Hydroxide ◽  
Supercritical Conditions ◽  
Hydrothermal Reactions

THE HYDRATION OF THE ALUMINATES OF CALCIUM: IV. HYDROTHERMAL REACTIONS OF TRICALCIUM ALUMINATE AND ITS HYDRATES

1943 ◽  
Vol 21b (4) ◽  
pp. 65-72 ◽  
Author(s):  
G. M. Harris ◽  
W. G. Schneider ◽  
T. Thorvaldson
Keyword(s):  
Calcium Hydroxide ◽  
Rectangular Plate ◽  
Hydrothermal Treatment ◽  
Saturated Steam ◽  
Partial Hydrolysis ◽  
Portland Cement Concrete ◽  
Steam Curing ◽  
Hydrated Alumina ◽  
Hydrothermal Reactions ◽  
Tricalcium Aluminate

A study was made of the hydrothermal reactions of tricalcium aluminate and its hydrates between 120° and 350 °C. Homogeneous samples of the hexahydrate of tricalcium aluminate may be prepared by treatment of the aluminate or its hexagonal hydrate in saturated steam at temperatures up to 150 °C. At temperatures of 250 °C. and above, under conditions favouring rapid hydrolysis, such as the addition of water to the anhydrous aluminate or hydroaluminate before autoclaving and rapid elevation of the temperature of the autoclave, crystals of calcium hvdroxide and of a hydroaluminate of lower lime–alumina ratio appear, usually mixed with the isometric hexahydrate. The new hydroaluminate occurs as rectangular, elongated, prismatic plates of low birefringence, refractive index 1.627, and has a lime–alumina ratio probably lower than 1.5. The hexagonal hydrate of tricalcium aluminate gives on hydrothermal treatment a better yield of the low-limed hydroaluminate than does the anhydrous aluminate. Only partial hydrolysis of the hexahydrate was obtained under any of the conditions used, but the hexagonal hydrate, autoclaved for 12 hr. at 350 °C., gave products composed almost entirely of the rectangular plate hydroaluminate and calcium hydroxide. No free hydrated alumina was found in the hydrothermal products.The rectangular plate hydroaluminate of calcium may also be prepared by the action of calcium hydroxide on hydrated alumina in saturated steam at 350 °C. It is therefore a stable product under those conditions.At temperatures below 250 °C. birefringent crystalline material is also formed on autoclaving tricalcium aluminate or its hydrates under conditions favouring hydrolysis. The experimental evidence indicates that one of the products is the birefringent calcium hydroaluminate formed on autoclaving the lower calcium aluminates at temperatures of 105 to 150 °C. (5).Precast Portland cement concrete products, when subjected to steam-curing, normally contain an excess of water, and the conditions for the formation of these birefringent hydroaluminates of calcium are therefore present. The same applies to the commercial steam-curing of other materials containing free lime and hydrated alumina. The formation of these hydroaluminates may explain the peculiar effect on the tensile and compressive strength of mortar and concrete test pieces produced by hydrothermal treatment at temperatures above 150 °C. (9).

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